(1) Field of the Invention
The present invention relates to a composite laminate for the manufacture of laminated stock materials used for packaging food, and paper cartons used for food trays, liquid cartons, paper cups, and the like. More particularly, it relates to a composite laminate comprising a biodegradable resin, and a method for the manufacture thereof. The items made from the composite laminate of the present invention are capable of being broken down by environmental action to which the items become subject as litter.
(2) Description of the Prior Art
Many stock materials and containers made from paper, paperboard, a plastic film, or a nonwoven plastic fabric used as substrate are used for a wide variety of applications.
These stock materials and containers, when made solely from a natural material such as paper, paperboard, cellophane and the like, do not perform well for functions such as watertightening, hygiene-retaining and heat-sealing of the contents; therefore, a multiple composite laminate is widely used for the manufacture of laminated stock materials and paper cartons. In this case, the natural material is used jointly with a plastic material, in particular a polyolefins such as polyethylenes and polypropylenes which are used as a sealant or coating layer of the multiple composite laminate to thereby create or reinforce the aforesaid functions.
Many items such as stock materials and containers are used as `one-trip` packaging materials and are destined, after a relatively short functional life, to arrive as a significant component of urban garbage. The amount of packaging materials as a component of urban garbage increases with an increase in the production of these items, and thereby the amount: of such garbage is reaching the maximum capacity of garbage disposal facilities. On the other hand, there are not enough garbage disposal facilities in the planning stage and thereby such components of waste or garbage constitute a mounting nuisance.
These bags and containers are made from a multiple composite laminate; even so, it has been proposed that they should be capable of being easily recycled after use or should be capable of being broken down biologically, i.e. should be biodegradable.
One of the major problems encountered with the recycling system is the difficulty in separating the composite into individual layers; therefore, the recycling system for stock materials and containers made from a multiple composite laminate has not been established.
Under such circumstances, biodegradable plastic materials have been developed. These materials are capable of undergoing degradation by the action of micro-organisms of the environment when they form a component of garbage disposed in a reclaimed land. Among them, straight-chain copolyesters of 3-hydroxybutyric acid and 3-hydroxyvaleric acid (hereinafter also referred to as "straight-chain 3-hydroxybutyric acid/3-hydroxyvaleric acid copolyester" or simply as "straight-chain copolyester") have functions comparable to those of conventional polyolefins such as polyethylenes and polypropylenes, for example, heat-sealing functions, in addition to biodegradability; therefore, the copolyesters of this kind are regarded as a promising substitute for materials used as sealant or coating layer.
For example, there is every reason to expect that a composite laminate comprising a natural type substrate, such as paper, paperboard, nonwoven rayon fabrics, cellophane films and the like, coated with the aforesaid straight-chain copolyester may be used for the manufacture of biodegradable stock materials and containers for food packaging purposes.
Generally, a co-extrusion coating technique is employed to laminate a substrate with a polyolefins such as polyethylenes and polypropylenes, from the standpoint of the costs of operation and processability.
The co-extrusion coating is carried out at high temperatures such as 290.degree. to 320.degree. C. Although straight-chain 3-hydroxybutyric acid/3-hydroxyvaleric acid copolyesters have a fairly good co-extrusion processability, they are liable to be subjected to thermal cracking at a temperature above 200.degree. C. When the co-extrusion coating is carried out at a temperature of 180.degree. to 190.degree. C., the straight-chain polyester is stable, but a composite laminate having a sufficient laminating strength to paper cannot be obtained due to a shortage of heat supply. In addition, when a thin film of the straight-chain copolyester with a thickness less than 50 .mu.m is desired, peripheral side areas (so-called "deckle edge") become too thick compared with those of polyethylenes and polypropylenes, resulting in an unstable operation.
In addition, although straight-chain 3-hydroxybutyric acid/3-hydroxyvaleric acid copolyesters, which are present in vivo as a product of metabolism of microorganisms, have a sufficient biodegradability, they are expensive materials.
Accordingly, it is an important factor in laminating the straight-chain copolyester that a minimum thin film can be laminated on a substrate in a stable state.
The applicant of the present invention has proposed a biodegradable composite laminate and a method for the manufacture thereof (Japanese Patent Publication No. 60-52950), the method comprising co-extrusion coating a thermoplastic elastomer and a peelable resin from the elastomer on a porous substrate, and thereafter stripping the peelable resin.